This invention relates to an image synthesizing apparatus in which two image signals are synthesized by extracting necessary parts from an image signal with use of a brightness level difference. In a television camera which generates image signal outputs for extraction, if it includes a system of varying the scan domain on the screen surface in order to change the size and shape of a projected image on the screen, the output image of an object, though it has a uniform brightness, exhibits a brightness variation depending on the image size and shape. Therefore, in such an apparatus in which extraction is performed by utilizing a brightness difference, the extraction quantity of the parts of an object having the same brightness does not become constant. This makes it difficult to extract desired parts from a synthesized image. Thus, it may happen that unnecessary parts are projected while necessary parts are not.